How stable will the snowpack be when it does start snowing?

Chris Radford - January 2, 2019

We just received this question from Mark who has been to some of our talks. Mark writes …

We are heading to the mountains from 1st – 10th Feb and the current weather conditions are quite concerning. It’s widely believed within our group that the warming and freezing of the current snowpack will make it difficult for the fresh snow to bond meaning when the snow finally arrives the avalanche rating is going to be very high.

Could you advise us how the current conditions will be impacted when it starts snowing?

I thought it would be best to post our answer here, so everyone can see it.

The technically correct answer is that we cannot be sure without making observations one or two days before the snow starts to fall (when is that???). There will be plenty of clues that we must study at that time from observations on the mountain and the avalanche bulletin published by Meteo France. But we can make offer some guidance here based on current conditions.

Instability is usually greatest following periods of consistently cold weather (especially clear nights) and this effect is most pronounced on north facing higher altitude slopes. This is because the cold temperatures cause the snow to degrade into what is known as ‘faceted grains’ or sort of cohesionless crystals (often referred to as ‘goblets’ by the French). This transformation happens faster also when the snowpack is relatively thin, as it is in many places in the Northern French Alps at the moment.

. You can see Henry demonstrating this type of effect in this video from 2016

Now we had this effect earlier in the season and we reported it in our snow reports. It was directly responsible for some avalanche accidents. However since then the snowpack suffered rainfall up to 2500m and prolonged mild weather.

This warming and freezing of the snowpack that we are witnessing now has resulted in a very stable base. This is now so pronounced that that avalanche danger rating is down to 1 and Meteo France declare click here

The snowpack is well stabilized with the absence of snowfall for 1 week and mild weather at altitude. The observed avalanches remain very small. The natural risks remain low, with a good refreeze at night. For skiers, the smooth and hard snow will experience strong winds. Look out for rare small wind slabs, possible in small, steep corridors at altitudes often above 3000-3300 meters, in the S to SE exposure.

We expect the condition of steep slopes will become more fragile with the cold and wind, danger of slipping.

So if there were new snow on Thursday, (there won’t be) then this snow would fall on a fairly solid base in many (but not all places) below approx 2500 metres, but it would come down on an unconsolidated and unstable base in many places above approx 2500 m as instability would be present in many places due to the new snow falling on this cohesionless base especially on North facing slopes for reasons described above. But there is no instability deriving from the warming and freezing – quite the opposite as the lower danger ratings below 2500 metres has consistently indicated over the last week or so!

At HAT we are on a campaign right now to dispel the myth that warming poses the greatest danger for skiers at risk of triggering an avalanche – also the idea that the warming and freezing creates a smooth layer that the snow doesn’t bond well to is a myth – it is the trigger and collapse or rupture of a weak weak layer that leads to accidental avalanches. So in fact, it is the opposite: persistent cold creates long-term permanent weak layers and instability. This is what we often warn about. Read our recent snow report to see how we discuss this issue. And this snow report as well

I hope this helps. But I cannot answer your question in relation to 1st February. That depends on the type of weather we get between now and then, the nature of any snowfall and the effect of the wind. Our weekly snow report published on Thursdays will address this. In particular the one we write on 31st January will be directly relevant to your trip.